This invention relates to a magnetooptic data storage medium of amorphous magnetic material and more particularly to a magnetooptic data storage medium including changeable and readable memory locations and unchangeable memory locations.
In recent years, the use of thin films of amorphous magnetic materials for thermomagnetic writing, erasing and megnetooptical reading has received particularly intensive study. This sort of optical memory system can be classified into the following categories, depending on data storage properties:
(1) it is readable only; PA1 (2) it can hold additional recordings and is readable immediately after writing; and PA1 (3) it is writable, readable and erasable.
Of these three different categories the last is most suitable for computer applications and typically comprises amorphous magnetic films as a storage medium.
Furthermore, the methods of writing for the magnetooptic storage medium developed to data are as follows: (a) a Curie point writing technique by which the temperature of a memory bit location is elevated above the Curie point where magnetizations are destroyed; (b) a compensation temperature technique which takes advantage of the coercivity falling when the memory bit location at about the compensation temperature is further heated; and (c) at temperature dependent coercivity technique relying upon the phenomemon where coercivity varies greatly with a temperature rise. Recording is achieved by applying a laser beam onto the memory bit location in the order of 1 um .phi. and thus varying magnetizations in light-activated domains due to temperature increases. Erasing recordings demands energy for restoring the original magnetizations, using the same optical system as for writing. This sort of amorphous magnetic material is well known as a changeable optical memory medium. Reversibility of the medium, however, results in erasing recordings upon malfunction or erroneous operation of a recording system and making data unstable due to fluctuations in the ambient temperature.